Practical_Antenna_Handbook_0071639586

C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 593
measurement of the reactive component of impedance. The Heath HD-Â1422 is, of course,
no longer made, but can sometimes be found as used equipment on amateur radio Web
sites or at hamfests. MFJ’s 204B antenna bridge is a current production item.
Figure 27.4 shows the circuit of a typical noise bridge. As before, the bridge consists
of four arms. An internally generated noise signal is applied to winding T 1a of a trifilar
wound transformer and coupled into the inductive arms (T 1b and T 1c ) of the bridge circuit.
The adjustable arm of the bridge is a series RC circuit consisting of two front panel
controls: a 200-ÂW potentiometer (R 8 ) and a 250-ÂpF variable capacitor (C 8 ). The potentiometer
sets the range (0 to 200 W) of the resistive component of measured impedance,
while the capacitor sets the reactance range (which varies with frequency). Capacitor C 6
in the “unknown” arm of the bridge balances the measurement capacitor. With C 6 in the
circuit, the bridge is balanced when C 8 is approximately in the center of its range. This
arrangement accommodates both inductive and capacitive reactances, which appear on
either side of the “zero” point (i.e., the midrange capacitance of C 8 ). When the bridge is
in balance, the settings of R and C reveal the impedance across the unknown terminal.
An internal source of broadband RF noise uses the spectrum produced by the avalanche
process inherent in a reverse-Âbiased zener diode, enhanced by the 1-ÂkHz square-Â
wave modulator (IC 1 and associated components) that chops the noise signal. An
amplifier, comprised of transistor stages Q 1 and Q 2 , boosts the noise signal to the level
needed by the bridge circuit.
a b c
Figure 27.4 Noise bridge circuit.